Location is key!
You have 4 subs aleady. if they are on the front wall, place them 1/4 into to room from the sidewalls, and then you have almost eliminated sidewall reflections. at the same time, build the boxes so the driver is 1/4 up from the floor. Also stack the 2 other on top, so those driver is 1/4 down from the roof. Boom, you now have a single bass aray setup, and you have basicly eliminated both sidewall and floor/celing reflections. All that is left is front and backwall reflections. Add som EQ and you might get away with just that. (depends where your null is, and if you can live with it.)